Short summary of my character: The party was in a position were they could you more spell casting fire power and a story element presented the opportunity to multicast as a sorcerer. It wasn't in the most ideal stage of character growth but the works with our overall story and character history; but since it wasn't planned the character will lose one Ability Score Improvement.

So the character has only two improvements remaining. I am considering taking the War Caster Feat and Skill Expert Feat. The Skill Expert Feat is something I want to take because the character will benefit adding proficiency to Arcana (again, the original plan wasn't to be the primary spell caster) which will support a gaming mechanic in our play. 

Where my question is, if I go with Skill Expert should I put my ability improvement into Dexterity (Score 13) or Charisma (Score 19). So do I max out my spellcasting modifier to a 20 (+5), or is the 19 more than sufficient and I should consider taking the DEX improvement in increase my AC. I have a decent AC combo now that brings me to about 16 now. I also have proficiency in DEX saves. 

Now there is another question, is War Caster overrated and not worth taking? So feel free to chime in on that.

I feel that War Caster is something I should take and I need the proficiency for Arcana, so Skill Expert is a must. Now, do I max out my spellcasting modifier or work to improve my AC?

If you aren't front-line, don't worry about AC.

Instead, gaining +1 to hit and on all your spell DCs could be more useful.

There are magic items to boost your DEX and AC. Bump your CHA up to 20.

My opinion: the dirty secret in dnd character building is that bee=lining your main stat to max isn't quite as important as we tend to think. 

Its a small but nice increase to spell save dc and attacks' chance to hit, but with how AC and the bonus to saving throws caps out and how much proficiency gives a character, its not some game breaking difference. 

I would grab the feats if you think the feats will be fun, and especially if they will be useful to your character. 

War Caster is always a good feat, but I would suggest it gets less and less useful for full casters as the game goes on. It depends on if you have a strategy to use a shield or otherwise fill both hands, what options you have for the opportunity attack, and whether or not the advantage on saving throws feels like it would be helpful. 

Ironically, I like War Caster better on half or 1/3rd casters who can exploit the blade cantrips and stack other things on them, who enemies want to run past to get to the dangerous spell casters, and who can get some advantage from using a shield. 

Unpopular opinion but Dex does so much more than just AC. I find striking 1st can often put the battle flow in your parties hand so increasing initative can be useful. Stealth for an ambush can be useful. Slight of hand to use a magic item possibly undetected can be useful. Ac if your front line gets bipassed or you get ambushed from behind can be helpful. That said you can't go wrong with a 20 on your main stat and increasing your casting succes. Both work.

I would recommend charisma. Also, war caster is good but slightly overrated. I would probably either go magic initiate, or alert. What Leaf_Onwind said about dex is true, but you could do this better ways. To sort out the initiative you can use alert, and the rest just hire a rogue.

At high CR AC doesn't matter than much, put it into your spellcasting

I have been in multiple 10+ campaigns and two that rose above 15th level. My best AC was 22 (27 with shield spell) and I regularly was getting hit by the enemies that mattered. Against and adult red dragon with a attack of +14 the high AC does help statistically, but HP and THP are way more important at the table. On the other side, hitting saves at higher game play is can be a little tougher, so having as high a DC as you can becomes clutch depending on your spell selection. I would probably arrange mechanical priorities as:

1 - anything to increase your DC (Shadowfell Shard even gives disadvantage to one target's save if you are a sorcerer)

2 - Anything that boosts HP (Taking the occasion hit without dropping is super helpful)

3 - Anything that boosts initiative (Disabling before the opponent gets a turn mitigates more damage to the party in the long run)

After that, it is more about how you play your character.